Schemes of wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) for transmitting multiplexed optical signals of different wavelengths have been adopted to further expand communication capacity. In combination with the WDM scheme, a phase modulation scheme of correlating information with the phase of an optical signal has also been used.
The phase modulation scheme includes differential phase shift keying (DPSK) by which information is correlated with the degree of phase shift of an input signal, and phase shift keying (PSK) for coherent communication in which information is correlated with a difference in phase between an output signal from a local oscillator in an optical receiving device and an input signal. Hereinafter, PSK for coherent communication will be referred to as simply “PSK”.
Under both phase modulation schemes in the receiving device, an intensity modulation (On-Off Keying (OOK)) signal is generated according to the difference in phase between a reference signal for phase comparison (an output signal from a local oscillator or a symbol just before an input signal) and an input signal. According to PSK, an OOK optical signal is generated by a mixer, such as optical coupler. According to DPSK, an OOK optical signal is generated by a one-symbol delay interferometer (demodulator).
In practical applications, DPSK is currently expected to be advantageous because DPSK is not easily affected by property-related fluctuations of an optical fiber serving as a transmission path and does not require a local oscillator. For example, a study of differential binary phase shift keying (DBPSK) and differential quadrature phase shift keying (DQPSK) has been in progress.
Among the above modulation schemes, DQPSK is characterized in that the expansion of the wavelength spectrum during modulation is small compared to other modulation schemes, such as DBPSK and OOK. For this reason, DQPSK enables WDM of a greater density and improved resistance to dispersion, and is thus expected to be a core technology aimed at long distance transmission systems having a larger capacity.
FIG. 14 is a block diagram of a conventional optical communication system. As depicted in FIG. 14, an optical communication system 1400 is an optical communication system that performs DQPSK communication for N multiplexed waves (wavelengths λ1 to λN). The optical communication system 1400 includes an optical transmitting device 1410 and an optical receiving device 1420. The optical transmitting device 1410 includes an optical transmitting circuit having N light sources 1411, N phase modulators 1412, and a wavelength multiplexer 1413.
The N light sources 1411 output beams of light of different wavelengths (λ1 to λN) to the N phase modulators 1412, respectively. The N phase modulators 1412 respectively modulate, by DQPSK, the beams of light output from the N light sources 1411. For example, the N phase modulators 1412 perform DQPSK modulation by shifting the phase of the light by four phase shift quantities (0°, 90°, 180°, 270°) corresponding to 2-bit information.
The N phase modulators 1412 output to the wavelength multiplexer 1413; modulated optical signals DQPSK(λ1) to DQPSK(λN). The wavelength multiplexer 1413 multiplexes the optical signals DQPSK(λ1) to DQPSK(λN) that are of different wavelengths and output from the N phase modulators 1412, and transmits the multiplexed optical signal DQPSK (λ1 to λN) to the optical receiving device 1420 via a transmission path such as an optical fiber.
The optical receiving device 1420 includes an optical receiving circuit having a wavelength separator 1421, N demodulators 1422, and N balanced receivers 1423. The wavelength separator 1421 demultiplexes (separates according to wavelength) the optical signal DQPSK (λ1 to λN) transmitted from the optical transmitting device 1410 through the transmission path and outputs the demultiplexed optical signals DQPSK(λ1) to DQPSK(λN) to the N demodulators 1422, respectively.
The N demodulators 1422 each include a delay interferometer, etc., and respectively extract from the optical signals DQPSK(λ1) to DQPSK(λN) output from the wavelength separator 1421, a positive-phase signal and a negative-phase signal for each channel (I-channel and Q-channel) as intensity modulation signals OOK (λ1, I-positive-phase) to OOK (λN, Q-negative-phase), and output the intensity modulation signals OOK (λ1, I-positive-phase) to OOK (λN, Q-negative-phase) to the N balanced receivers 1423, respectively.
The N balanced receivers 1423 are provided corresponding to each wavelength and channel, each performing balanced reception of a positive-phase signal and a negative-phase signal for a corresponding wavelength and channel among the intensity modulated signals OOK (λ1, I-positive-phase) to OOK (λN, Q-negative-phase) to demodulate the received signals. The present embodiments relate to an optical receiving circuit included in the optical receiving device 1420.
FIG. 15 is a block diagram of a configuration of part of a conventional optical receiving circuit. As depicted in FIG. 15, a conventional optical receiving circuit 1500 for DQPSK-based WDM includes N modulators 1520 disposed downstream from a wavelength separator 1510. The optical receiving circuit 1500 is thus generally configured to operate in such a way that the N demodulators 1520 receive optical signals DQPSK(λ1) to DQPSK(λN) separated according to wavelength by the wavelength separator 1510 (see, e.g., Japanese Patent Application Laid-Open Publication No. 2005-094287).
FIG. 16 is a block diagram of a configuration of a part of another conventional optical receiving circuit. As depicted in FIG. 16, a conventional optical receiving circuit 1600 for DQPSK-based WDM includes four wavelength separators 1620 disposed downstream from a demodulator 1610. The optical receiving circuit 1600 is thus configured to operate in such a way that four wavelength separators 1620, respectively separate according to wavelength, positive-phase signals and negative-phase signals for each channel (I-positive-phase to Q-negative-phase) extracted by the demodulator 1610 from an optical signal DQPSK (λ1 to λN) (see, e.g., Japanese Patent Application Laid-Open Publication No. 2006-246471).
The configuration of the optical receiving circuit 1500 of FIG. 15, however, requires the same number of demodulators 1520 as the number of multiplexed waves N, which may amount to about 100 in DWDM. The configuration of the optical receiving circuit 1500 of FIG. 15, therefore, invites an increase in the size of the optical receiving circuit 1500 as the number of multiplexed waves N increases. This leads to a problem in that the cost of the optical receiving circuit 1500 increases.
According to the configuration of the optical receiving circuit 1500 of FIG. 15, a delay rate at each of the demodulators 1520 equivalent in quantity to the multiplexed waves N has to be set for each corresponding wavelength. This leads to a problem of a further increase in the cost of the optical receiving circuit 1500. The configuration of the optical receiving circuit 1600 of FIG. 16 poses a problem in that optical cables between the wavelength separators 1620 and the balanced receivers cross each other.
FIG. 17 is a block diagram of an optical receiving circuit to which the optical receiving circuit of FIG. 16 has been applied. As depicted in FIG. 17, plural wavelength separators 1620 are disposed downstream from the demodulator 1610. In this configuration, among signals output from the demodulator 1610, a positive-phase signal and a negative-phase signal to be input to the same balanced receiver 1710 (hereinafter “corresponding positive-phase signal and negative-phase signal”) are separately input to different wavelength separators 1620.
As a result, the corresponding positive-phase signal and the negative-phase signal are output from positions separated from each other, which causes one set of optical cables 1720 transmitting the corresponding positive-phase signal and negative-phase signal intercross between the wavelength separators 1620 and balanced receiver 1710. This arises in a problem that uniforming the optical lengths of the set of optical cables 1720 transmitting the corresponding positive-phase signal and negative-phase signal is difficult.
When the optical lengths of the set of optical cables 1720 transmitting the corresponding positive-phase signal and negative-phase signal are not uniform, the positive-phase signal and the negative-phase signal are input to the balanced receiver 1710 at different times relative to each other. This leads to a problem of a decline in the precision of demodulation by the balanced receiver 1710.
For example, if a difference in the timing of input of the corresponding positive-phase signal and negative-phase signal to the balanced receiver 1710 becomes 0.2 times (or more) the modulation period, optical signal noise ratio (OSNR) deteriorates (see, e.g., “IEEE Photonics Technology Letters”, Sep. 2003, Vol. 15, No. 9, pp. 1282-1284).
For example, when DQPSK-based communication at a transmission rate of 40 Gb/s is performed, to keep the difference in input timing of the corresponding positive-phase signal and negative-phase signal to the balanced receiver 1710 equal to or lower than 0.2 times the modulation period to avoid deterioration of the OSNR, an optical length difference between the optical cables 1720 must be kept within a range of ±2 mm. The precision of the length of a commercial optical fiber is approximately +100 mm to −0 mm, which does not meet the demanded precision for optical investigations.
If the optical cables 1720 between the wavelength separators 1620 and the balanced receiver 1710 cross each other and become complicated, another problem arises in that efficient integration of the optical receiving circuit 1600 and maintenance work, such as replacement of the optical cable 1720 between the wavelength separator 1620 and the balanced receiver 1710 and addition of another channel (wavelength), become difficult.
FIG. 18 is a block diagram of optical connectors used for a conventional optical receiving circuit. As depicted in FIG. 18, because optical signals output from the demodulator 1610 are separately input to different wavelength separators 1620 in the optical receiving circuit 1600 of FIG. 16, the optical cables 1720 transmitting these signals must be individually connected with optical connectors 1820. Because one set of optical cables 1720 transmitting the corresponding positive-phase signal and negative-phase signal are arranged crossing each other, the optical cables 1720 are connected individually with optical connectors 1830.
If the number of multiplexed waves is 32, the optical receiving circuit 1600 needs 1 optical connector 1810 upstream from the demodulator 1610, 4 optical connectors 1820, and 128 optical connectors 1830, thus requiring 133 optical connectors in total. The optical receiving circuit 1600 thus needs a multiplicity of optical connectors, which increases the size of the optical receiving circuit 1600, leading to a problem of an increase in the cost of the optical receiving circuit 1600.
The above problems also arise when PSK-based communication is performed. When an optical receiving circuit for PSK has a configuration such that plural mixers (equivalent to demodulators for DPSK) are disposed downstream from a wavelength separator, the optical receiving circuit needs mixers equivalent in quantity to the number of multiplexed waves N, which leads to a problem of an increase in the size and cost of the optical receiving circuit.
When an optical receiving circuit for PSK is configured to have wavelength separators downstream from a mixer, one set of optical cables 1720 transmitting a corresponding positive-phase signal and negative-phase signal cross each between the wavelength separators and a balanced receiver. This poses a problem of lower demodulation precision and difficulty in maintenance.